Cellulose and potato starch are typically used. Cellulose is a carbohydrate polymer (meaning it consists of lots of glucose molecules linked together) derived from plants. It's used for structure in green plants and it not digestible by humans so it acts as a fiber. Potato starch is also a carbhydrate polymer which is basically just a component of potato flour. The only real difference between cellulose and starch is how the glucose molecules (naturally) link together. That link is actually what makes starch digestible and cellulose non-digestible. I would consider both natural since they do not contain any type of chemicals and are derived from plant sources.
Natamycin is considered natural but I guess that's up to the individual: http://en.wikipedia.org/wiki/Natamycin